Changes in Injection Well Capacity During Testing and Plant Start-up at Ngatamariki

Abstract

The Ngatamariki geothermal field is situated 17 km north east of Taupo, New Zealand, and in 2013 Mighty River Power commissioned an 82 MW power station on the field consisting of four Ormat Energy Converter units. Production is from three deep wells in the center of the field and injection is back into four deep wells split between the North and South. Four of these wells were drilled during a campaign in 2012, including three injection wells. In addition to the normal completion testing, each of the new injection wells was injected with cold river water for several weeks to determine potential for stimulation. Prior to plant start-up, proved injection capacity was less than 50% of what was required to run the plant at full generation. Data from the stimulation testing phase constrained a simple analytic model of well stimulation based on Grant et al. (2013) and this model was used to predict injectivity evolution during plant start-up. Modeling predicted the injection wells would stimulate up to the required capacity during the staged start-up of the four Ormat units with minimal generation losses, and accordingly the decision to drill an additional injection well was deferred. During plant start-up the injection wells did stimulate within the predicted range. This paper presents the injectivity index data during injection stimulation testing with cold river water and during plant operation when the wells were injected with 90°C geothermal brine. The injectivity evolution is discussed in the context of the geological structure and fracture properties of the rocks, micro-seismic activity and recent developments in well stimulation modeling.